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    <script>
        //Boolean类型：有两个固定的值可以用，true false
         var flag1=true;
         var flag2= false;
         console.log(flag1);
         console.log(flag2);

         var flag3;   // 默认值：undefine
         console.log(flag3+'aabbcc');   //undefinedaabbcc
         console.log(flag3+1);  //NaN

         /* console.log(flag4+'aabbcc'); */  //报错   flag4 is not defined
      
         console.log(true + 1);  // 2  true对应的数字是1
         console.log(false + 1); // 1  false对应的数字是0

         var name=null;
         console.log(name);   //null是一个值
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